iPod nano USB battery charger

No questions about this, Apple designs are remarkable. Their products look great, are easy to use, and generally work great. The price? Yes, the price is high, but not prohibitively high.

What makes me angry is how they try to monetize even on small details, sometimes by bending the standards.

The problem

For example the nice gadget that is the iPod nano; as most today devices, it requires an USB port for charging. Unlike most other devices, it refuses to charge from a no-name USB charger, and requires an Apple special charger (a $29 device).

Most of the users don’t bother to buy one, and charge the iPod battery using a computer port, that works as expected.

However, especially when I travel, I prefer to use a separate device, a 4 USB port charger, that I also use for my phone, my GPS, and generally all other devices.

Connecting the iPod to this charger made the screen turn on, so it detects the external voltage, but the charging process refuses to start.

The solution

With “a little help from my friends” (thank you, Lix!), it looks like the iPod checks the voltage on the 2 unused USB signal lines, and if not around +2.5V, it refuses to start. Since no-name USB chargers usually leave the USB signals unconnected, they are not able to charge the iPod nano.

The solution is easy: use your preferred no-name USB charger, and add two resistive divisors, one on each USB signal. This accounts to 4 x 22K resistors [Note 1], connected between the the USB signal lines and the Ground and +5V lines.

After adding these resistors to one of the ports of my charger, the iPod nano was no longer picky and started to charge the battery! (The battery icon, located in the upper right corner, changed from an empty battery to a full one, containing a thunderbolt sign).

[May 22, 2012 Update]

[Note 1] Regarding the values of the resistors, further studies reveled that the values of the resistors tell to some Apple devices the power capabilities of the charger, for example an original iPhone charger uses 49.9K to ground and 75K to Vcc.